1. Field of the Invention
The present invention relates to a process for producing a magnetic recording medium in which a smoothing coating layer is provided on a non-magnetic support, and a magnetic layer is formed directly on the smoothing layer or via a non-magnetic layer.
2. Description of the Related Art
In the field of magnetic disks, 2 MB MF-2HD flexible disks employing Co-modified iron oxide are standard equipment in personal computers. However, the volume of data to be handled is rapidly increasing at present, such a capacity is not sufficient, and there has been a desire for an increase in the capacity of flexible disks.
In the field of magnetic tapes also, accompanying the recent spread of mini computers, personal computers, and office computers such as work stations, magnetic tape for recording computer data as an external memory medium (so-called backup tape) has been intensively investigated. When a magnetic tape is put into practice for such an application, there has been a strong requirement for an increase in the recording capacity in order to achieve an increased recording capacity and a reduction in dimensions accompanying the reduction in the dimensions of computers and the increase in the information processing capacity.
Conventionally, as magnetic recording media, those formed by coating a non-magnetic support with a magnetic layer in which iron oxide, Co-modified iron oxide, CrO2, a ferromagnetic metal powder (MP), or a hexagonal ferrite powder is dispersed in a binder are widely used. Among these, the ferromagnetic metal powder and the hexagonal ferrite powder are known to have excellent high density recording characteristics. In the case of disks, with regard to a high capacity disk employing a ferromagnetic metal powder having excellent high density recording characteristics, there are the 10 MB MF-2TD and the 21 MB MF-2SD, and with regard to a high capacity disk employing hexagonal ferrite, there are the 4 MB MF-2ED, the 21 MB floptical disk, etc., but they are not yet adequate from the viewpoints of capacity and performance. Under such circumstances, there have been a large number of attempts to improve the high density recording characteristics. For example, high density recording with a high capacity of 100 M to 120 M such as the LS-120 and ZIP has been achieved and, furthermore, high density recording having an areal density of 0.2 Gbit/inch2 or higher is now being required. Moreover, because of requests to reduce the access time, there is a trend for the disk rotational speed to increase.
In magnetic recording media having such high density and high rotational speed or high transfer speed, in order to maintain stable record and playback thereof, their transport characteristics and durability are required to be higher than the conventional media. The prior art, which has been proposed mainly in order to improve the recording density and the transport durability in coated type magnetic recording media, is explained below.
For example, JP-A-6-52541 (JP-A denotes a Japanese unexamined patent application publication) discloses a magnetic tape in which the average projection height of an abrasive on the surface of a magnetic layer is 15 nm or less, the head wear and the head contamination are improved, and a balance is achieved between the electromagnetic conversion characteristics and the durability. JP-A-6-12651 discloses a magnetic disk in which the Ra of a magnetic layer is 15 nm or less, the distribution of projections of 30 nm or higher in the magnetic layer is from 125,000 to 250,000/mm2, and the amount of a lubricant in the magnetic layer is given.
JP-A-6-309650 discloses a magnetic recording medium which contains 8 to 30 parts by weight of a lubricant relative to 100 parts by weight of a magnetic powder, and the number of projections having a height between that of the highest projection in a magnetic layer and 20 nm less than that is 400 to 2,500/mm2, that is, a magnetic recording medium in which the transport stability is maintained by specifying the amount of lubricant in the magnetic layer and the population density of projections having a specific height on the surface of the magnetic layer and, in particular, a magnetic disk.
Conventionally, magnetic heads (induction type magnetic head) employing electromagnetic induction as the operating principle are widely used. However, when they are used in the high density record and playback field a limit starts to be seen. That is, in order to obtain a large playback output, it is necessary to increase the number of windings of a coil of a playback head, but this increases the inductance and increases the resistance at high frequency, and as a result the playback output decreases, which is a problem.
Recently, a playback head employing MR (magnetoresistance) as the operating principle has been proposed, its use in hard disks, etc. has started, and in JP-A-8-227517 its application to magnetic tape is proposed. The MR head gives a playback output several times that of the induction type magnetic head, and since it does not use an induction coil, equipment noise such as impedance noise is greatly reduced, and by reducing the noise of the magnetic recording medium it becomes possible to obtain a large S/N ratio. In other words, by reducing the magnetic recording medium noise, which had been hidden by equipment noise, record and playback can be carried out well, and the high density recording characteristics are outstandingly improved.
However, the MR head has the problem that it generates noise (thermal noise) under the influence of microscopic heating; in particular, it has the problem that when it hits a projection present on the surface of a magnetic layer, the noise suddenly increases and continues, and in the case of digital recording the problem can be so serious that error correction is impossible. This problem of thermal noise becomes serious in a magnetic recording medium used in a system in which a recorded signal having a recording density of 0.5 Gbit/inch2 or higher is replayed.
In order to reduce such thermal noise, it is important to control the surface properties of the magnetic layer, and as a measure therefor the surface properties of a support have been controlled. For example, one in which a polyurethane coating layer is provided on a support (ref. Japanese Registered Patent Nos. 2938548 and 2938549) and one in which a layer formed from a compound that polymerizes by exposure to radiation is provided on a support (ref. JP-B-5-57647 (JP-B denotes a Japanese examined patent application publication)) have been proposed.
However, in the former prior art, the support thus produced is expensive, and the smoothness is inadequate.
Furthermore, in both the former and the latter prior art, it is assumed that the smoothed support is temporarily wound into a roll, and the roll thus wound is unwound in a separate step of applying a functional layer such as a magnetic layer by coating or vapor-deposition. However, the smoothed support is difficult to handle and easily creases, in particular while winding it, thus giving rise to the problem that the productivity and the yield greatly decrease.